1. Field of the Invention
The invention relates to a retaining device for absorber tubes. The invention also relates to an arrangement made up of absorber tubes and a retaining device.
2. Description of Related Art
Parabolic trough collectors in solar thermal power plants have parabolically curved mirrors, which can extend over a length of several hundred meters. Arranged in the focal line of these parabolic mirrors are absorber tubes, which have a metal tube and a cladding tube. Such absorber tubes are known from DE 102 31 467 B4, for example.
In order to reach the required length, it is necessary to join a plurality of absorber tubes together with one another. The metal tubes are welded together with one another for this purpose. Arranged in each junction is a retaining device, which, on the one hand, is fastened to the support structure of the parabolic trough collector and, on the other hand, engages with the absorber tubes.
Known from WO 2010/142666 A2 is such a retaining device, which has posts or supports that extend outward from the interior of the parabolic mirror and, at the upper end of which, is arranged a tube clamp that clasps the metal tube in its junction. This tube clamp has a main part made of two outward extending pieces of sheet metal spaced apart from each other, at which two tube clamp halves are arranged by means of a hook-like hinge so as to move without becoming detached. Situated at the free ends of the tube clamp halves is a screw bolt connection. Arranged on the inside of the tube clamp halves are projections, which rest against the metal tube. These point-like rest supports have the advantage that heat losses due to thermal conduction can be reduced. However the heat losses at the junction of the metal tube are still too large in spite of this measure.
Thermal shielding materials in the form of insulating materials, for example, to reduce heat losses at the open regions of the absorber tube have been used previously. However, the installation of a ribbon-type insulation material prior to mounting of the retaining clips has the drawback that the force fit between the retaining clip and the absorber tube is not ensured and that there is a risk that, owing to the tracking movement of the collector as well as owing to the longitudinal movement of the insulating material caused by thermal expansion of the absorber tube, the insulating material is pressed out of its original position and there is no longer a force fit of the retaining clips. Moreover, when the insulating material is permanently subject to penetration of moisture—for example, during prolonged periods of inclement weather—corrosion problems can arise at the weld seams of the metal tubes lying beneath the insulating material.
Such insulating materials generally have a reflectance of markedly less than 0.5, particularly for application in the infrared region at wavelengths greater than 3 μm. Reflectance is understood to mean the ratio of reflected intensity to incident intensity.
Known from EP 1 690 047 B1 is a solar collector, in which, in the junction of two absorber tubes, a radiation shielding element of length L is arranged, at the end of which a mirror collar is provided. The radiation shielding element has a larger radius of curvature than the glass cladding tube and covers the entire free space between the two cladding tubes of the absorber tubes. On account of the great distance of the radiation shielding element from the metal tubes, this arrangement has the drawback that a large amount of thermal radiation occurs from the metal tubes onto front-side components of the absorber tubes, such as, for example, the glass-metal transition elements, and heats them.